1. Field of the Invention
The present invention relates to a microwave, and more particularly to a microwave oven for radiating microwaves generated from a magnetron into a cavity to heat and cook foodstuff disposed therein.
2. Description of the Prior Art
FIG. 1 is a schematic sectional view of a microwave oven according to a first embodiment of the prior art, where the microwave oven includes a magnetron 1 and an antenna 2. The antenna 2 is disposed with a waveguide 3 and a feeder holds 4.
The microwave generated from the magnetron 1 is radiated into a cavity 5 via the antenna 2, i.e., via the waveguide 3 and the feeder hole 4, and heats and cooks foodstuff 6.
There are a lot of antennas as illustrated in FIGS. 2a and 2b, where the antenna in FIG. 2a is called a slot antenna 7. The slot antenna 7 is formed such that sides of the waveguide 3 are blocked by conductor plates 9 to which a slot feeder hole 10 is positioned at right angle, where the microwaves generated from the magnetron 1 are radiated into the cavity 5 through the waveguide 8 and the slot feeder hole 10.
However, there is a problem in the slot antenna 7 in that directivity of the microwaves radiated into the cavity 5 is bad due to sudden changes of impedance at the slot feeder hole 10 and narrow area of the slot feeder hole 10.
Accordingly, a known microwave oven has adopted an aperture antenna 11 which (FIG. 2b) is an improvement over the slot antenna 7. The aperture antenna 11 has an advantage in that impedance matching between the waveguide 12 free space is good, and directivity of the microwaves radiated into the cavity 5 is excellent. There is another advantage in the aperture antenna 11 in that structure thereof is simple and manufacturing is easy, which is why the aperture antenna is widely used.
The aperture antenna, as illustrated in FIG. 2b, is formed with a larger feeder hole 13 than the waveguide 12, such that the microwaves generated from the magnetron 1 are radiated into the cavity 5 via the waveguide 12 and the feeder hole 13.
These types of antenna, as illustrated in FIG. 3, concentrate energy to one direction and radiate microwaves, so that microwave radiation power density in the cavity differ according to directions.
At this time, microwaves emitted to a central front area of the feeder hole at the antenna are called main radiation waves and microwaves radiated at a wider angle then the main radiation waves are called auxiliary radiation waves. The radiation width of the microwaves emitted into the cavity is defined by an angle having the radiation power density of microwaves 3 dB lower than a maximum radiation power density of the microwaves.
An antenna with a good directivity has a radiation width of microwaves emitted into the cavity at 1.degree. and a radiation power density of auxiliary radiation waves is lower by 30 dB to 50 dB than the maximum radiation power density.
Accordingly, in order to obtain an antenna having a good directivity, it is necessary to widen the feeder hole (two sides a and b of the feeder hole) compared with the wavelength of the microwaves. On the contrary, if the feeder hole is small, directivity becomes lower, such that the microwaves emitted into the cavity take a spherical thereby be evenly radiated to all directions in radiation power density thereof.
However, in a microwave oven, an antenna with a poor directivity of microwaves radiated into the cavity obtains a better uniform heating efficiency than that of a better directivity because electromagnetic field distribution in the cavity is even.
Furthermore, microwaves of linearly polarized wave type are emitted into the cavity from an antenna of the conventional microwave oven thus described, as illustrated in FIG. 4, such that the linearly polarized waves proceed with formation of linear polarization.
Still furthermore, molecules in the foodstuff also perform the linear polarization motion when the linearly polarized waves the proceed with formation of linear polarization pass through the foodstuff, as illustrated in FIG. 5. The molecules in the foodstuff thus perform the linear polarization motion to generate heat by themselves and to heat the foodstuff.
However, there is a problem in the conventional microwave oven thus described in that the microwaves radiated into the cavity are linearly polarized waves which proceed with formation of linear polarization, such that molecules of the foodstuff heated and cooked by the linearly polarized waves also performs the linearly polarization motion to thereby reduce an absorption efficiency of energy absorbed by the foodstuff in comparison with the circularly polarized waves.
There is another problem in that the microwave emitted into the cavity which are linearly polarized waves cannot obtain a uniform heating efficiency due to better directivity than the circularly polarized waves.